An electric power steering apparatus is, as is well known, an apparatus which is configured to assist in driver's steering force using driving force of a motor; nevertheless, a conventional electric power steering apparatus is configured in most cases to include one driving circuit for one motor. However, in recent years, it can be known that an electric power steering apparatus is mounted on a whole category of wheeled vehicles, thereby revealing that, if assist functions stop due to a failure or fault of the electric power steering apparatus, it is close to impossible for the driver to operate the steering wheel, causing difficulties in running the wheeled vehicle itself. For this reason, the need is intensified to continue the assist, depending on content of the fault, as much as possible even when the electric power steering apparatus becomes faulty.
For dealing therewith, conventionally, an electric power steering apparatus is proposed in which two sets of three-phase motor coils are provided for one motor, and two sets of driving circuits are included to separately control each of those three-phase motor coils (for example, refer to Patent Document 1). The conventional electric power steering apparatus disclosed in Patent Document 1 is so arranged that, when a fault is detected in one system constituted of a set of three-phase motor coils by a fault detection means, the control is continued by decreasing a motor instruction value using a remaining normal system. At this time, because the motor instruction value is decreased, assist force by the motor becomes smaller, so that a driver can recognize that the fault occurs in the electric power steering apparatus.